Mechanisms of tropical precipitation biases in climate models

We investigate the possible causes for inter-model spread in tropical zonal-mean precipitation pattern, which is divided into hemispherically symmetric and anti-symmetric modes via empirical orthogonal function analysis. The symmetric pattern characterizes the leading mode and is tightly related to the seasonal amplitude of maximum precipitation position. The energetic constraints link the symmetric pattern to the seasonal amplitude in cross-equatorial atmospheric energy transport AET(0) and the annual-mean equatorial net energy input NEI0. Decomposition of AET 0 into the energetics variables indicates that the inter-model spread in symmetric precipitation pattern is correlated with the inter-model spread in clear-sky atmospheric shortwave absorption, which most likely arises due to differences in radiative transfer parameterizations rather than water vapor patterns. Among the components that consist NEI0, the inter-model spread in symmetric precipitation pattern is mostly associated with the inter-model spread in net surface energy flux in the equatorial region, which is modulated by the strength of cooling by equatorial upwelling. Our results provide clues to understand the mechanism of tropical precipitation bias, thereby providing guidance for model improvements.

Automated summary

Through investigating the causes of inter-model spread in tropical zonal-mean precipitation pattern, it is found that the symmetric pattern is closely related to the seasonal amplitude of maximum precipitation position and is affected by clear-sky atmospheric shortwave absorption. The inter-model spread in net surface energy flux in the equatorial region, modulated by equatorial upwelling cooling, is the main factor associated with the inter-model spread in symmetric precipitation pattern.